Method of fabricating gallium arsenide burris FET structure for optical detection

ABSTRACT

A GaAs FET structure with a high electric field region, or active region, contacted by source, gate and drain electrodes is provided which can be used for high speed optical detection or for microwave oscillator optical injection locking. The device provides for efficient coupling of optical radiation into the active region through an opening in a semi-insulating substrate used to support the device. A buffer layer between the active region and the substrate prevents leakage current to the substrate, permits a larger illumination window for improved optical coupling and provides mechanical support for the FET detector. GaAs photodetectors are also provided by eliminating the gate electrode.

The Government has rights in this invention pursuant to Contract No.N00173-78-C-0192 awarded by the Department of the Navy.

This is a division of application Ser. No. 133,183, filed Mar. 24, 1980,now U.S. Pat. No. 4,346,394.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a GaAs FET structure having a high electricfield region which can be used for high speed optical detection or formicrowave oscillator optical injection locking. More particularly, theinvention relates to a device configured such that optical radiation isefficiently coupled into the high electric field region.

2. Description of the Prior Art

Gallium arsenide field effect transistors (FET's) are well-known; see,e.g., Vol. 24, Institute of Electrical and Electronic Engineers,Microwave Theory and Technology, pp. 279-300 (1976). However, thesedevices are designed for optimal microwave performance only. Therefore,they are not necessarily suitable for efficient optical coupling.

Other investigators have employed conventional FET's as opticaldetectors; see, e.g., Technical Digest, International Electron DevicesMeeting, pp. 120-123 (1978), Vol. 13, Electronics Letters, p. 193 (Mar.15, 1977) and Vol. 19, Japanese Journal of Applied Physics, pp. L27-L29(1980). However, these FET's also were optimized only for microwaveperformance.

SUMMARY OF THE INVENTION

In accordance with the invention, an integrated optical photodetectorcomprises:

(a) a substrate of semi-insulating III-V semiconductor material, havingat least one opening therein;

(b) a buffer layer of mixed III-V semiconductor material, supported onat least a portion of the substrate, at least one portion of whichbuffer layer is exposed by the opening; and

(c) a detector supported on the buffer layer and operably associatedwith the opening. The detector includes an active region and a pair ofelectrodes contacting the active region. The buffer layer is undoped andhas an indirect bandgap. The bandgap is larger than that of the activeregion.

The device is fabricated by a process comprising:

(a) forming a buffer layer of mixed III-V semiconductor material on atleast a portion of a substrate of semi-insulating III-V semiconductormaterial;

(b) forming at least one opening in the substrate to expose at least oneportion of the buffer layer; and

(c) forming a detector on the buffer layer operably associated with theopening by a process including:

(i) forming an active region on the buffer layer, and

(ii) forming a pair of electrodes in contact with the active region.

The advantage of the device is that incoming optical radiation can beefficiently coupled into the active region (high electric field region)through the opening in the semi-insulating substrate, thereby optimizingoptical characteristics.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a cross-sectional view of a GaAs FET in accordance with theprior art; and

FIG. 2 in cross-section depicts a GaAs FET in accordance with theinvention.

DETAILED DESCRIPTION OF THE INVENTION

The discussion that follows relates specifically to GaAs devicesincluding mixed (AlGa)As regions, useful for high speed opticaldetection, for microwave oscillator optical injection locking and othermicrowave-optical interactions. Such GaAs devices typically operate atan optical wavelength of about 0.6 to 0.9 μm and at microwavefrequencies ranging up to at least about 5 GHz. However, it will beunderstood that the invention is suitable for other III-V devices suchas InP, used in similar applications but in a different opticalwavelength range (about 1 to 1.5 μm for InP).

In order to achieve optical injection locking, mixing and high speeddetection using GaAs FET amplifiers or oscillators, it is desired toimprove the optical coupling efficiency of GaAs FET's. FIG. 1 depicts aconventional GaAs FET structure. The structure comprises asemi-insulating substrate 10, such as GaAs, typically having aresistivity of about 10⁷ ohm-cm. The substrate supports an active region11 of typically III-V semiconductor material, here, n-GaAs doped toabout 10¹⁷ /cm³ and having a thickness ranging from about 0.5 to 1 μm.The purpose of the active region is to provide a region for thegeneration and control of carriers. Electrodes 12 and 13 are in directcontact with the active region. For an FET device, electrodes 12 and 13serve as source and drain regions, respectively, and gate electrode 14contacts a portion of active region 11 and is spaced apart from thesource and drain electrodes. In the case of a photoconductor, gateelectrode 14 would be absent. In either case, the front side of thedevice is illuminated by radiation 15.

The prior art devices operate through the photoconductive process bywhich injected photons produce a change in the conductivity of theactive layer. This results in an induced photoconductive current.

The disadvantage with prior art FET's is the small active area availablefor optical absorption due to the presence of gate electrode 14. This isespecially true for FET's with a short channel length (the distancebetween electrodes 12 and 13).

In accordance with the invention, an FET structure with improved opticalcoupling efficiency is provided. The structure is depicted in FIG. 2,which shows a structure similar to that in FIG. 1, except that anundoped gallium aluminum arsenide layer 16, called the "buffer" layerherein, is provided between substrate 10 and active region 11. Anopening 17 is provided through the substrate for efficiently introducingoptical radiation 15' into the structure from the backside.

Semiconductor layers 11 and 16 are conveniently grown by conventionalliquid phase epitaxy (LPE) techniques. The FET electrode pattern isdefined as usual. Following these procedures, at least one well oropening 17 is etched in the semi-insulating substrate. Although only oneopening is shown, it will be understood that a plurality of openings,each associated with a discrete device, are formed in the substrate. Theposition of the opening is aligned carefully with the FET channel usingan infrared microscope. In the case of using undoped Al_(x) Ga_(1-x) Asas the buffer layer, a selective etchant such as superoxol (NH₄ OH--H₂O₂) is preferably used, since it etches only GaAs and not (AlGa)As.Non-selective etchants may alternatively be used for a portion of theetching if desired; however, the final etching must use a selectiveetchant in order to control the thickness of the buffer layer.

The composition of the buffer layer 16 for a GaAs FET is given by Al_(x)Ga_(1-x) As, where x is at least 0.37. The value of x should be as highas possible, since this permits employing shorter wavelengths, which inturn implies a wider wavelength span (larger bandgap) and also lowersmobility, which has the effect of reducing the leakage current. On theother hand, x should not be too high, since traps can be created at theinterface, resulting in long lifetimes which can decrease the highfrequency performance. Further, high x compositions become difficult togrow. Below the value of x of 0.37, the mobility increases rapidly.Above a value of about 0.7, growth problems result. Preferably, x rangesfrom about 0.4 to 0.5 and most preferably is about 0.4.

The thickness of the buffer layer is at least about 5 μm in order toprovide mechanical support. If the thickness is too great, however, thedevice leakage current may be too high and may prevent pinchoff of theFET and reduce the efficiency of the detector. Accordingly, thethickness of the buffer layer should not exceed about 25 μm andpreferably ranges from about 5 to 10 μm.

The opening 17 is formed by employing a preferential etchant that stopsat the buffer layer. For devices employing a semi-insulating substrateof GaAs and a buffer layer of (AlGa)As, an example of such an etchant issuperoxol, as mentioned above, which comprises an aqueous solution of29% NH₄ OH and 30% H₂ O₂. The solution is made up to a pH ranging fromabout 8 to 9. As is well-known, the pH determines the ratio of NH₄ OH toH₂ O₂. A pH greater than about 9 results in an undesirably fast etch,while a pH less than about 8 results in pitting of the semiconductorsurface. Preferably, the pH is about 8.5.

Prior to forming opening 17 in the substrate, the substrate ispreferably thinned in order to reduce the etching time and to avoidhaving to fabricate a large window opening, since the superoxol etchanttypically etches at an angle of about 15° to 40°.

The thickness of the active layer 11 is of the same order as the opticalattenuation depth, which varies with wavelength. For devicescontemplated herein, that thickness ranges from about 0.5 to 1 μm. Inall other respects, the detector portion of the device is provided byconventional processing using well-known techniques.

The source and drain contacts (electrodes 12 and 13) are convenientlyformed by depositing layers of goldgermanium (typically about 88% Au-12%Ge), nickel and gold, followed by heat treating, and the gate contact(electrode 14) is formed by depositing aluminum. These depositions arewell-known in the art and form no part of this invention. Individualdetectors are isolated by forming means. The mesa etch is superoxol.

While only one device is depicted in FIG. 2, it will be understood thata plurality of devices are fabricated in the substrate. The finisheddevices may be separated into individual detectors, as by slicing anddicing the substrate, employing well-known techniques, or formed intoone- or two-dimensional arrays of detectors by suitable interconnection.

The FET detector functions like a photoconductor except that (1) thegate can control the dark current and (2) the generation of carriers canchange the depletion layer width, thus changing the drain-sourcecurrent. Further, phototransistor action may be achieved by connecting aresistor (not shown) in series with the gate. Thus, if the gate acts asa photodiode, a voltage is induced across the gate and a correspondingchange in the drain-source current occurs.

The foregoing discussion has been concerned with employing an FET inconjunction with light detection. A photodetector may alternatively beemployed, using electrodes 12 and 13 and omitting gate electrode 14.Such photodetectors function as a photoconductor. The photoconductivityis changed by incident optical radiation and thus an induced current isgenerated.

While the FET and the photodetector structures are per se well-known, itis the combination of these structures with light detection and thegeometry employed that is considered novel.

In the structure disclosed herein, the buffer layer has severalfunctions. It serves as an optical window, transparent for photon energylower than the bandgap energy of the buffer layer. The buffer layer alsoreduces GaAs surface recombination loss, provides mechanical support andserves as the stop for selective etching during fabrication. A primeadvantage of the structure of the invention is that the entire channellayer can be uniformly illuminated and the active layer is large becausethe gate electrode does not interfere with incident optical radiation.By not doping the buffer layer, its conductivity may be kept low. Bychoosing an appropriate percent of aluminum such that the buffer layerbecomes indirect bandgap, the carrier mobility in the layer may befurther reduced. Since the bandgap energy of the buffer layer is higherthan that of the channel layer, the heterojunction also serves toconfine charge carriers in the channel layer. Thus, leakage current dueto the presence of the buffer layer will be minimized.

Advantageously, the structure of the invention permits coupling ofoptical fibers, such as fiber pigtails, to the device through opening17.

EXAMPLES

A device similar to that depicted in FIG. 2 has a source-drain spacing,or channel length, of 15 μm and a gate length of 5 μm. The active region11 is 0.5 μm thick and doped with n-type carriers (Sn) to about 10¹⁷/cm³ (mobility about 3,500 cm³ /V-sec). The buffer layer 16 of Al_(x)Ga_(1-x) As is 5 μm thick, with x=0.4 and a background n-type doping ofabout 5×10¹⁵ /cm³ (mobility about 100 cm² /V-sec).

The opening 17 in substrate 10 is a square about 50 μm×50 μm. The sourceand drain contacts 12 and 13 are formed by evaporating 88% Au-12% Ge(1,500 Å), Ni (1,000 Å) and Au (2,000 Å) and heat treating at 460° C.for 30 sec; the gate electrode 14 is formed by evaporating Al (2,000 Å).

The device evidences the following typical characteristics: DC lightsensitivity of about 2 to 10 mA/mW, RF cutoff frequency greater than 4GHz, with an RF sensitivity of about 0.1 mA/mW, drain-source current(I_(DS)) of about 10 to 20 mA, drain-source voltage (V_(DS)) of about 2to 5 V, transconductance of about 2 to 4×10⁻³ mho and gate voltage(V_(g)) of about 0 to -5 V.

What is claimed is:
 1. A process for fabricating an integrated opticalphotodetector on a substrate of semi-insulating III-V semiconductormaterial comprising:(a) forming an undoped buffer layer of mixed III-Vsemiconductor material on at least a portion of said substrate, saidbuffer layer having an indirect bandgap; (b) forming at least oneopening in said substrate to expose at least one portion of said bufferlayer; and (c) forming at least one detector on said buffer layeroperably associcated with at least one opening by a processincluding:(i) forming an active region of III-V semiconductor materialon said buffer layer, said buffer layer having an indirect bandgap, thevalue of which is larger than that of said active region, and (ii)forming a pair of electrodes in contact with said active region.
 2. Theprocess of claim 1 comprising:(a) forming on a substrate ofsemi-insulating GaAs a buffer layer of undoped Al_(x) Ga_(1-x) As, wherex ranges from about 0.37 to 0.7; (b) forming at least one opening insaid substrate to expose at least a portion of said buffer layer; and(c) forming at least one detector on said buffer layer operablyassociated with said at least one opening by a process including: (i)forming an active region of n-type GaAs on said buffer layer, and (ii)forming a pair of electrodes in contact with said active region.
 3. Theprocess of claim 1 or 2 in which a gate region is formed on a portion ofsaid active region, said gate region being formed approximately centeredopposite said at least one opening and spaced apart from said pair ofelectrodes.
 4. The process of claim 2 in which x ranges from about 0.4to 0.5.
 5. The process of claim 4 in which x is about 0.4.
 6. Theprocess of claim 2 in which said at least one opening is formed by apreferential etchant.
 7. The process of claim 6 in which saidpreferential etchant comprises an aqueous solution of ammonium hydroxideand hydrogen peroxide having a pH ranging from about 8 to
 9. 8. Theprocess of claim 7 in which said pH is about 8.5.
 9. The process ofclaim 2 in which the thickness of said buffer layer ranges from about 5to 25 μm.
 10. The process of claim 9 in which the thickness of saidbuffer layer ranges from about 5 to 10 μm.
 11. The process of claim 2 inwhich the thickness of said active region ranges from about 0.5 to 1 μm.